This invention relates generally to electrical propulsion systems for diesel electric locomotives equipped with alternating current traction motors and, more particularly, to a method for controlling locomotive operation without use of a direct current sensor.
In a conventional diesel electric locomotive, a thermal prime mover (typically a 16 cylinder turbo-charged diesel engine) is used to drive an electrical transmission comprising a synchronous generator that supplies electric current to a plurality of alternating current (AC) traction motors whose rotors are drivingly coupled through speed reducing gearing to the respective axle wheel sets of the locomotive. The generator typically comprises a main three-phase traction alternator, the rotor of which is mechanically coupled to the output shaft of the diesel engine. When excitation current is supplied to field windings on the rotating rotor, alternating voltages are generated in three-phase armature windings on the stator of the alternator. These voltages are rectified to produce a controlled amplitude DC voltage on a DC link which is connected to DC terminals of one or more inverters. The inverters invert the DC voltage to a controlled frequency of AC power to be supplied to the field windings of the AC traction motors. The effective AC excitation frequency produced by the inverters controls the speed of the AC motors with power being controlled by pulse width modulation of the AC waveform.
In normal motoring operation, the propulsion system of the diesel electric locomotive is so controlled as to establish a balanced steady state condition wherein the engine driven alternator produces, for each discrete position of a throttle handle, a substantially constant optimum amount of electrical power for the traction motors.
The propulsion control system conventionally includes means for limiting or reducing alternator output voltage as necessary to keep the magnitude of the DC link voltage and the magnitude of DC link current from exceeding respective predetermined safe maximum levels or limits. Typically, both the DC link voltage and the DC link current are monitored with appropriate sensors which provide both a feedback control of the propulsion system operation and also serve to prevent over-voltage and/or over-current conditions. At low locomotive speeds, the traction motor armatures are rotating slowly so that their back EMF is low. A low alternator voltage can now produce a maximum motor current which in turn produces the high tractive effort required for acceleration. On the other hand, the alternator voltage magnitude must be held constant and at a high magnitude whenever locomotive speed is high since the traction motor armatures are rotating rapidly and have a high back EMF and the alternator voltage must be high to produce the required load current.
In addition to providing current feedback signals for controlling alternator operation during motoring (a propulsion mode of operation of the locomotive in which the AC motors are powered to drive the locomotive in either a forward or reverse direction), the DC link current sensor is also used to control engine operation during self-loading (a test mode with the locomotive at rest in which alternator power is dissipated in a dynamic braking resistance grid coupled to the DC link) to test engine, alternator and associated controls. The DC link current sensor also provides inputs to a failure detection circuit for identifying short-circuit currents associated with a shoot-through condition typically caused by switching device failures in one of the inverters or any other shorted condition at or reflected onto the DC link. Accordingly, the DC current sensor at the DC link has been considered to be a necessary feature for locomotive systems.
A detriment of the DC link current sensor is its relatively high cost. Further, failure of the sensor may require shutting down the associated locomotive. The DC link current sensor is unusual because of the need to monitor a wide range of DC current, e.g., anywhere from a few tens of amperes up to several thousand amperes. Additionally, the sensor is connected in a relatively high voltage circuit, e.g., the nominal DC link voltage may be about 1400 volts but can spike up above 2000 volts. Accordingly, it is desirable to provide a method for overcoming the cost and failure disadvantages associated with use of a conventional DC link current sensor.